Question

(A) The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close. Explain the reason. [3 Marks]
(B) (i) Explain how you would separate a mixture of oil and vinegar in brief. [2 Marks]
(ii) How can we separate salt from salt solution? [1 Mark]
(C) Answer the following questions:
(1) Give two examples of changes that are both irreversible and desirable. [1 Mark]
(2) Justify the characteristics of physical change with the help of examples. [3 marks]

Answer 1

(A) $\bullet$ Intermixing of particles of two different types of matter is called diffusion. [1 mark]
$\bullet$ The rate of diffusion increases with increase in temperature and decreases with decrease in temperature. [1 mark]
$\bullet$ Therefore, the smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close. [1 mark]

(B) (i) Vinegar and oil are immiscible in each other. Vinegar being denser than oil settles at the bottom of the container. They can be separated using a separating funnel. [1 mark]
This mixture should first be transferred to a separating funnel and left undisturbed to settle down. Then, the stopcock of the funnel should be slowly opened, thus allowing the denser liquid (vinegar) to flow out into a beaker. Once the vinegar is fully collected, the stopcock should be closed so that no oil flows out. After this, the oil can be collected in a separate beaker. [1 mark]
(ii) A salt solution is a mixture of salt and water. Salt can be separated from the salt solution by evaporating the water.
The mixture is heated so that the water evaporates and the salt remains in the container, thus separating from the mixture. Water can be obtained back from water vapour using condensation. [1 Mark]

(C) (1) The two changes that are both irreversible and desirable are the cooking of food and the growth of an infant. [1 mark]
(2) (i) No new substances are formed during physical change.
Eg: When ice is heated, it melts and turns into water. When the water is heated further, it turns into steam. The steam condenses and becomes water when it cools. The water forms into ice as it cools further. Ice, water, and steam molecules, always have two hydrogen atoms and one oxygen atom. As a result, we can conclude that no new substances are formed. [1 mark]
(ii) Physical changes are generally reversible.
Eg: When the wax is heated, it turns into a liquid. However, when liquid wax cools, it solidifies. [1 mark]
(iii) There is no change in weight during physical change.
Eg: Make a salt solution by dissolving 20g of salt in water. It will end up with 20g of salt after the water evaporates. [1 mark]